Hydrocarbons (hereinafter, referred to as HC), carbon monoxide (hereinafter, referred to as CO) and nitrogen oxides (hereinafter, referred to as NOx) , contained in exhaust gas discharged from internal combustion engines such as cars, boilers and industrial plants, form the main cause of air pollution. Especially, since NOx form the cause of acid rain, the development of a technique for removing NOx from exhaust gas is considered to be an urgent task.
Conventionally, in the case of exhaust gas from gasoline engines, such as those of cars, a method has been known in which exhaust gas is treated by a so-called three way catalytic converter using platinum or other elements so as to remove NOx together with HC and CO. This method is extremely effective when the air-fuel ratio (hereinafter, referred to as A/F) is set in the vicinity of the stoichiometrical air-fuel ratio (A/F=14.6).
In recent years, lean-burn engines have been gained attention as a means for improving fuel economy and for reducing CO.sub.2. However, such engines have a greater A/F than the stoichiometrical air-fuel ratio, and form an atmosphere in which an excessive amount of oxygen exists in the exhaust gas (hereinafter, referred to as an "oxidizing atmosphere") Since an excessive amount of oxygen exists as compared with the amount required for completely burning unburned ingredients such as HC and CO in the exhaust gas, it is difficult to remove NOx through reduction by using a normal three way catalytic converter.
Moreover, in the case of Diesel engines whose exhaust gas forms an oxidizing atmosphere, a method is known in which a reducing agent, such as ammonia, hydrogen or carbon monoxide, is used to remove NOx from exhaust gas from a Diesel engine that forms a stationary source of generation, such as a boiler.
In this method, however, an additional device for adding the reducing agent and a special device for recovering and treating the unreacted reducing agent are needed; this makes the entire equipment more complex and bulky, and the resulting problem is that this method is not applicable to engines that form a movable source of generation such as cars.
In order to solve the above-mentioned problems, various catalysts have been proposed for removing NOx in an oxidizing atmosphere.
However, no conventional methods have successfully provided an NOx removing catalyst which can decompose and remove NOx in exhaust gas effectively even in an oxidizing atmosphere, which is superior in heat resistance and durability under high temperatures, and which can exert a catalytic activity in a wide temperature range.
As some of those NOx removing catalysts, for example, aluminosilicate with exchanged transition-metal ions, such as copper ions, (see Japanese Laid-Open Patent Publication No. 125250/1985 (Tokukaisho 60-125250), Japanese Laid-Open Patent Publication No. 100919/1988 (Tokukaisho 63-100919) and the specification of the U.S. Pat. No. 4,297,328) or metallo-aluminosilicate (see Japanese Laid-Open Patent Publications No. 127628/1991 (Tokukaihei 3-127628) and No. 229620/1991 (Tokukaihei 3-229620)), and silico-aluminophosphate (see Japanese Laid-Open Patent Bublication No. 293049/1990 (Tokukaihei 2-293049) have been proposed.
However, these so-called ion-exchange zeolite catalysts require high temperatures to remove NOx and consequently have reduced effects at low temperatures, and are inferior in heat resistance to the point that their NOx decomposing capability is extremely reduced when exposed to high-temperature exhaust gas; accordingly, these catalysts have not been successfully put into practical use.
Moreover, as an NOx removing catalyst for use in an oxidizing atmosphere, a catalyst having iridium deposited on a fire-resisting inorganic oxide such as alumina has been disclosed (see Japanese Examined Patent Publications No. 54173/1981 (Tokukousho 56-54173) and No. 13328/1982 (Tokukousho 57-13328)). However, in the embodiments described in these patent publications, only examples using the oxygen concentration in exhaust gas not more than 3 volume % are shown, and no consideration was given to NOx purifying capability and heat resistance with respect to exhaust gas from diesel engines and lean-burn engines that contains oxygen not less than the above-mentioned amount.
Furthermore, catalysts having iridium deposited on a support such as zeolite and crystalline silicate have been proposed (see Japanese Laid-Open Patent Publications No. 296870/1994 (Tokukaihei 6-296870), No. 80315/1995 (Tokukaihei 7-80315) and No. 88378/1994 (Tokukaihei 7-88378)). However, with respect to conditions of the durability tests for these catalysts, those tests were merely carried out in an reducing atmosphere of exhaust gas, and no consideration was given to durability and heat resistance in an oxidizing atmosphere such as exhaust gas from diesel engines, lean-burn engines and gasoline engines of the fuel-direct-injection type.
Moreover, catalysts having iridium deposited on a support made of a material such as a metallic carbide and a metallic nitride have been proposed (see Japanese Laid-Open Patent Publications No. 31173/1994 (Tokukaihei 6-31173), No. 31884/1995 (Tokukaihei 7-31884), No. 246337/1995 (Tokukaihei 7-246337, No. 33845/1996 (Tokukaihei 8-33845) and No. 71422/1996 (Tokukaihei 8-71422).
However, the examples of the above-mentioned Patent Publications merely show maximum NOx removing rates, and with respect to temperature ranges at which the maximum NOx removing rates are exerted, nothing is clarified except for the case in which light-off characteristics are shown. The light-off characteristic shows the exhaust-gas-purifying characteristic at various exhaust gas temperatures.
Judging from the example having the light-off characteristic, it is the temperature range exceeding 350.degree. C. that the activity for removing NOx appears, and the NOx purifying activity is extremely reduced in the temperature range not more than 350.degree. C.
In addition, another disadvantage of these conventional catalysts is that after having been used for a long time, the temperature at which the NOx purifying activity rises is greatly shifted toward the high-temperature side. Further, since metallic carbides and metallic nitrides are expensive, the cost increases. Consequently, the catalysts, described in the above-mentioned Patent Publications, have merely narrow temperature ranges in the activity for removing NOx, and also have high costs.
Consequently, at present, no conventional methods have successfully provided an NOx removing catalyst which can decompose and remove NOx in exhaust gas effectively even in an oxidizing atmosphere, which is superior in heat resistance and durability under high temperatures, and which can exert a catalytic activity in a wide temperature range at low costs.
Moreover, with respect to car engines, there is a tendency to set the temperature of exhaust gas at a low level with a view to achieving a more efficient combustion and a lower fuel consumption; therefore, there is an increasing demand for a method for reducing HC, CO and NOx from exhaust gas even at lower exhaust gas temperatures.
Furthermore, Laid-Open International Patent Publication No. WO 93/08383 discloses a catalyst which oxidizes and adsorbs NOx in an oxidizing atmosphere, while discharging NOx in a reducing atmosphere, and an exhaust-gas-purifying method using such a catalyst.
In this method, however, sulphur oxides, contained in exhaust gas, are irreversibly adsorbed simultaneously with NOx, and the resulting disadvantage is that the NOx purifying capability deteriorates with time due to the adsorption.
Consequently, at present, the above-mentioned conventional methods have failed to successfully provide an exhaust-gas-purifying catalyst which can decompose NOx efficiently and remove it from exhaust gas even in an oxidizing atmosphere, which is superior in heat resistance under high temperatures, which can avoid degradation in performance due to poisoning from sulphur oxides, etc., and which exerts a catalyst activity for reducing HC, CO and NOx in exhaust gas in a wide temperature range, especially, in a low temperature range, and an exhaust-gas-purifying method using such a catalyst.
The objective of the present invention is to provide an exhaust-gas-purifying catalyst which is superior in oxidizing activity for HC and CO, reduces NOx efficiently not only in a reducing atmosphere but also in an oxidizing atmosphere, has high heat resistance and high durability, and also exhibits its activity in a wider range, and a purifying method for exhaust gas.
Moreover, a catalyst on which a complex oxide of iridium is deposited together with noble metals such as platinum, rhodium and palladium has been proposed (see Japanese Laid-Open Patent Publication No. 277369/1993 (Tokukaihei 5-277369). Although this method can improve heat resistance of a catalyst, its NOx removing capability is low in high-temperature ranges, as compared with the other temperature ranges.
As described above, at present, it has not been successful to develop any catalyst which decomposes and removes NOx from exhaust gas efficiently even in an oxidizing atmosphere, is superior in heat resistance and durability at high temperatures, and exhibits a catalytic activity in a wide temperature range at low costs.
Moreover, in each of the embodiments of the above-mentioned Patent Publications, although the maximum NOx removing rate obtained by the use of the corresponding catalyst is listed, the exhaust gas temperature at which the maximum NOx removing rate was obtained is not given. Furthermore, with respect to the catalyst whose light-off characteristic is shown.
The present invention has been devised to solve the above-mentioned conventional problems, and its objective is to provide an exhaust-gas-purifying catalyst which removes NOx efficiently not only in a reducing atmosphere but also in an oxidizing atmosphere, exhibits an NOx-removing activity in a wide temperature range, is superior in heat resistance and durability, and can suppress the temperature range at which the NOx-purifying performance is exerted from shifting toward the high-temperature side, and also to provide a process for purifying exhaust gas.